Repeating earthquakes and interplate coupling along the western part of the North Anatolian Fault

The Sea of Marmara accommodates segments of the North Anatolian Fault (NAF) in Turkey and remains the only part of the western NAF that has not ruptured during the last century. At its nearest, the segment is ~20 km from Istanbul, the largest city in Turkey. Thus, it is important to understand the locking state of the fault, since it illuminates the strain accumulation rate along the fault segments, which in turn is an important input parameter in seismic hazard studies. To infer the interplate locking state, we used repeating earthquakes that indicate fault creep in the surrounding area using long-term (April 2005 to May 2013) seismic observations at 12 broadband seismic stations operated by Bogazici University Kandilli Observatory and Earthquake Research Institute. We defined repeating earthquakes from waveform coherences that were >0.95 for 40-second-long waveforms of the vertical component. Using the selection procedure, we found 21 repeating earthquakes with magnitude 2.3 to 3.2 that are grouped into 9 sequences. They are distributed along the main NAF, comprising three groups of activity, one group in the Sea of Marmara and a group either side to the east and west. The three groups are located near the boundary of previous large earthquake ruptures, suggesting relatively weak coupling there. We also estimated the fault creep rate from the cumulative slip of the repeating earthquakes using a scaling relationship between repeating earthquakes' moment and slip. The slip rate for these three groups are similar (3–4 cm/yr) and comparable to, albeit slightly higher than, those expected from global plate models (~2.4 cm/yr). This suggests relatively weak locking around the groups. The relocation results of the repeating earthquake hypocenters in the Sea of Marmara suggest the creep is occurring at 10 to 20 km depth. These results suggest heterogeneous coupling in the segment.